In general, precursor fiber bundles specially designed for carbon fiber production are used in carbon fiber production processes. These precursor fiber bundles are commonly wound up on a bobbin or folded and stored in boxes in the precursor fiber bundle supply equipment. Precursor fiber bundles pulled out of the precursor fiber bundle supply equipment are commonly supplied to a calcination step that comprises an oxidizing step and a carbonizing step.
To continue the calcination of precursor fiber bundles for a long period of time to continue carbon fiber production for a long period of time, therefore, the front end portion of the precursor fiber bundle pulled out from the precursor fiber bundle supply equipment has to be joined by some means with the tail end portion of the precursor fiber bundle that is passing through the calcination step. By joining the end portions of these precursor fiber bundles in their length direction, it becomes possible to supply the precursor fiber bundles continuously to the carbon fiber production process, consequently leading to improvement of the operation of the process.
There is a known method in which length-directional end portions of respective two polyacrylonitrile-based precursor fiber bundles, which are used as precursor fiber bundles for carbon fiber production, are joined by applying pressurized fluid jets to interlace the fibers (see Patent Literature 1).
However, though it is actually possible to join the end portions of precursor fiber bundles by this method, the fiber density will be too high in the fiber joint portion formed, giving rise to the problem of runaway of the oxidization reaction caused during the oxidizing step by the heat generated from the precursor fiber bundles themselves. Accordingly, there have been accidents involving thermal destruction and burnout of the fiber joint portion. To prevent the breakage of the fiber joint portion from being caused by heat accumulation, there is the means of lowering the temperature of the oxidizing step. If the temperature of the oxidizing step is lowered significantly, however, a longer time will be required for carrying out the oxidizing step, leading to a considerable decrease in the productivity for the desired carbon fibers.
If the precursor fiber bundles are composed of a large number of filaments, the pressurized fluid jets emitted from jetting nozzles will not be able to cover the entire precursor fiber bundles, and the precursor fiber bundles will not be interlaced at the filament level, but instead divided into sub-bundles that are interlaced. If such sub-bundles are formed unevenly in the fiber joint portion, the fiber density will increase locally to accelerate heat accumulation. In addition, sufficient interlacement will not be achieved in the fiber joint portion, leading to a smaller binding strength between the precursor fiber bundles. As a result, the fiber bundles will become unable to resist the tension caused during the process, leading to rupture or slippage of the bundles in the fiber joint portion.
For instance, as a known solution to this problem, two polyacrylonitrile-based precursor fiber bundles may be joined by means of a connection medium (joint fiber bundle) composed of oxidized fibers that do not generate heat (see Patent Literature 2). Though this method can reduce the quantity of heat accumulation, however, the heat in the joint portion cannot be removed sufficiently, and breakage of the yarn may still occur easily in the joint portion where the fiber density has increased.
Therefore, the furnace temperature has to be decreased as the fiber joint portion passes through the oxidizing step. In addition, the oxidized fibers that constitute the joint fiber bundle and the fibers that constitute the polyacrylonitrile-based precursor fiber bundle are different in the way they are unraveled in their respective bundles, and accordingly, the fibers that constitute the polyacrylonitrile-based precursor fiber bundle and the oxidized fibers that constitute the joint fiber bundle are not commingled sufficiently and fail to be interlaced uniformly. This can cause slippage of these fiber bundles, leading to forced shutdown of the oxidizing furnace for fire prevention purposes.
There is another known method in which instead of interlacement and joining achieved by pressurized air, the fiber bundles are divided into several sub-bundles in their end portions, and joined by braiding the sub-bundles together (see Patent Literature 3). In this case, the joined bundles form nodes, which are tightened to increase the fiber density in the joint portion, leading to heat accumulation that causes breakage of the yarn. Furthermore, there will be a large variation in the binding strength among the sub-bundles in the joint portion, and a stress is concentrated on those sub-bundles with a smaller binding strength, causing breakage of the sub-bundles starting with those with a smaller binding strength.
In addition, there is a proposal of polyacrylonitrile-based fiber bundles for carbon fiber production that are produced by oxidizing the end portions of precursor fiber bundles to form oxidized fiber bundles having a density of 1.30 g/cm3 or more, and joining together precursor fiber bundles with such end portions by interlacing and integrating the fibers in the end portions to form a joint portion (see Patent Literature 4). In this case, though breakage of the yarn due to heat accumulation in the joint portion tends to be reduced, a special apparatus is required to make the end portions of the precursor fiber bundles to oxidized fibers, leading to a lower productivity.